Three-Body Spectrum of 18C and its Relevance to r-Process Nucleosynthesis
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The 18C spectrum has been studied in a three body n + n +16C model that includes deformation and the 2+ excitation of the 16C core as well as Pauli projection of forbidden states. The 16C – n interaction employed in this study has been fitted to reproduce the experimental spectrum of 17C. The calculations show that two neutron separation energy in 18C in consistent with three-body structure of this nucleus and predict more states bound with respect to three-body decay. The comparison of their position to known excited states in 18C is discussed. These calculations suggest also that a few states may exist in astrophysically relevant region between the 17C+n and 16C + 2n decay thresholds. The most important of them is 1− as it can give a large E1 resonant contribution to 17C(n, γ)18C neutron capture. The calculations also suggest that a virtual s-wave state may exist above the 17C + n threshold that can give rise to non-negligible M1 contributions to the 17C(n, γ)18C reaction rate. The presence of these states in the 18C spectrum can lead to an increased 17C(n, γ)18C reaction rate, which can significantly influence the abundances of uranium and thorium synthesized in the r-process in the supernovae explosions.
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